1. Field of the Invention
The present invention relates to a photothermographic material. More specifically, the invention relates to a photothermographic material which is suitable for recording images for medical use.
2. Description of the Related Art
In recent years, in the field of films for medical imaging, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light-sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light-sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are known. In particular, photothermographic materials generally have an image forming layer in which a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. This system has been described in many documents. Further, the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system using photothermographic materials that has been made commercially available.
Photothermographic materials are classified into two types according to the production process thereof. One is a solvent coated-type photothermographic material which is produced by organic solvent coating, and the other is an aqueous coated-type photothermographic material which is produced by using an aqueous solvent and polymer latex as a main binder. The latter process is advantageous with respect to environmental suitability and mass production, because it does not require a step for recovering used solvent or the like and does not require a complicated production apparatus therefore. In the case of the solvent coated-type photothermographic material, vaporization of organic solvent remaining in the material often occurs during thermal development accompanied by deterioration of a working environment thereby. In contrast, the aqueous coated-type photothermographic material does not have such a problem.
Thermal developing processing does not require the processing solutions used in wet developing processing, and has an advantage in that processing can be carried out easily and rapidly. However, on the other hand, the photothermographic materials contain all chemicals necessary for image formation in coated layers in advance, and therefore, these chemicals may be degraded by reacting with each other, or with oxygen or water in the air. Therefore, there are problems such as deterioration in performance during storage after production of the photothermographic material and prior to use.
Especially, with respect to photothermographic materials used for medical use, it is an important task to obtain favorable color tone stably because image tone exerts influences on diagnostic ability.
To adjust the image tone, it is known in the art to use a dye or pigment in the image forming layer or back layer.
It is well-known in the art that the polymer latex used for the aqueous coated-type photothermographic material contains a small amount of ammonium, because an ammonium compound such as ammonium persulfate is commonly used for a polymerization initiator, and after the polymerization process, an ammonium compound is used as a pH adjusting agent to improve storage stability of the latex liquid. It is disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 2000-010229, 2002-296724, and 2002-258435 that photographic performance is improved when the ammonium salt included in the polymer latex is reduced or eliminated. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein.